Laser diodes, also known as an injection lasers or diode lasers, are commonly used in optical fiber systems, communication systems, compact disc players, laser printers, remote-control devices, intrusion systems, and the like to provide coherent radiation in response to applied current.
It is desirable to control the power output of a laser diode with an automatic power control (APC) circuit to enhance reliability and stability. Conventional automatic power control circuit 100 used to control the optical power output (PO) of common-anode laser diode 105 is shown in FIG. 1. The function of control system 100 is to increase the current flowing through laser diode 105, ILD, in response to a decrease in PO, and to decrease the current flowing through laser diode 105 in response to an increase in PO, such that a constant PO is maintained. Briefly, in FIG. 1, a servo-control loop is formed by drive device 160 (here an NPN transistor), laser diode (LD) 105, monitor photodiode (PD) 110, laser diode optical power output set resistor (R1) 140, and control operational amplifier 120. Laser diode 105 and monitor photodiode 110 may be packaged together as a so-called transmitter optical sub-assembly (TOSA), 115. The optical power output (PO) of laser diode 105 is proportional to the laser diode current (ILD) supplied by NPN drive transistor 160. Photodiode 110 monitors the optical power output of laser diode 105 and produces a proportional output current (IPD). The monitor photodiode current provides negative feedback to control op amp 120. The result is that the voltage at node 125, the inverting input of op amp 120, is stabilized to VREF, the voltage at the positive or non-inverting input of op amp 120 provided in control loop 100 by voltage source 135. In the configuration shown, monitor photodiode current is set by IPD=VREF/R1. Operational amplifier 120 drives the base of transistor 160 to satisfy the servo loop by increasing or decreasing the laser diode current until the voltage across R1 140 is substantially equal to VREF.
If the value of resistor 140 decreases, then the photodiode current, and hence laser diode optical power output, will increase to satisfy the servo loop. Likewise, if the value of resistor 140 increases, then the photodiode current and laser diode optical power will decrease. Accordingly, in the embodiment shown in FIG. 1, resistor 140 is used to set and adjust the desired optical power output of the laser diode.
The magnitude of resistor 140 and the voltage at node 130 play a role in determining the stability and usability of control system 100. That is, resistor 140 must be adjustable over a wide enough resistance range to accommodate the full range of anticipated values for IPD and ILD, preferably with a high degree of resolution, while keeping control system 100 stable and operative. However, IPD and ILD can vary significantly across diode types and applications. Further, resistor 140 must have sufficient thermal, or other parameter, stability to adjust ILD during operation to maintain temperature, or other operating parameter, stability.
Consequently, resistor 140 must have a large adjustment range and good thermal stability and typically is realized with a mechanical potentiometer or a digital potentiometer. A temperature-controlled non-volatile digital potentiometer can theoretically provide both initial calibration and temperature compensation. However, providing resistor 140 with these operating characteristics may be prohibitively expensive.
Further, large variations in laser diode and photodiode characteristics, such as output optical energy, wavelength and the like, as well as variations in optical power output requirements according to application or status, result in tradeoffs between the achievable PO adjustment range and resolution for a given resistor 140. Typical laser diode parameters and variations can be found, for example, in “Modulating VCSELs: Honeywell Application Sheet” (Honeywell Inc.; Freeport, Ill.) http://content.honeywell.com/vcsel/technical/006703—1.pdf, hereby incorporated by reference, and in HFE419x-521 datasheet “Fiber Optic LAN Components—LC Connectorized High-Speed VCSEL 2.5 Gbps” www.honeywell.com/sensing/VCSEL, hereby incorporated by reference. Accordingly, to achieve sufficient dynamic range for calibration, and adequate resolution for temperature compensation, prior art automatic power control systems for laser diodes typically employ one of the following solutions: (1) a one-size-fits-all approach for resistor 140, degrading system performance due to the range versus resolution limitations of potentiometers, or (2) replacing a resistor 140 with several resistors or with dynamically switchable resistors, or adding a second potentiometer, which increases manufacturing complexity and cost.
Further, when providing temperature compensation, devices typically use a discrete time sampling technique. Jitter in the amount of compensation applied may when the temperature, or other discrete-time measured parameter, is close to the threshold between two adjacent digital results, and the measured value varies stochastically between those two results. Since the amount of compensation applied depends on the temperature, or other measured parameter, the compensation is subject to the same jitter. Further, quantization noise may cause the digital result to vary between two adjacent results when the measured parameter is in between two adjacent results. Quantization noise is generally equal to ±½ least significant bit (LSB).
In general, there is a need for an automatic power control system that is relatively insensitive to laser diode and photodiode parameter variations, while providing control operation over a wide range of optical power output requirements. Such a system should provide laser diode current control, including control for a variety of laser diode configurations. There is a further need for a compensation system and method that is relatively free from jitter due to electronic and/or quantization noise.
The present invention provides such a control and compensation system.